Semiconductor saturable absorbers have found applicability as modelocking elements in solid state lasers for generating extremely short duration optical pulses. These pulses are commonly called ultrashort pulses because they exhibit pulse widths in the picosecond and sub-picosecond ranges. Ultrashort optical pulses are useful for high speed signal processing and data communications.
The saturable absorber allows passive modelocking of a laser when the absorber, which is a nonlinear element, is placed either within the lasing optical cavity or in an optical cavity, coupled and external to the lasing cavity. Saturable absorbers act as shutters to incident radiation because they can change their opacity as a function of the intensity of the incident radiation at a particular wavelength. A saturable absorber can absorb all weak incident radiation. As the intensity of incident radiation reaches a sufficiently high level known as the saturation intensity, incident radiation is permitted passage through the saturable absorber. In general, attenuation caused by the absorber is relatively low because the absorber is saturated into a transparent state at the desired wavelength.
Semiconductor saturable absorbers have been fabricated for narrowband and broadband response. Bulk semiconductor material and multiple quantum well heterostructures have been used for narrowband absorption applications while specially graded bandgap multiple quantum well heterostructures have been developed for broadband applications. In the quantum well realizations of such absorber devices, the quantum well heterostructure has been grown on a semiconductor quarter-wave stack reflector. In another embodiment known as an anti-resonant Fabry-Perot saturable absorber, a thin film oxide partial reflector stack was deposited on the quantum well heterostructure to form a Fabry-Perot etalon with the semiconductor quarter-wave stack reflector. For the latter device, the saturable absorber element (MQW) responds to radiation at wavelengths in the anti-resonant portion of the Fabry-Perot etalon response characteristic. This device produces weak coupling with the laser cavity and introduces less loss than other multiple quantum well devices used for modelocking the laser. By the same token, the anti-resonant Fabry-Perot saturable absorber requires significant additional device processing and optimization for its realization.